A conventional parking assistance system determines a distance of a motor vehicle to objects in its surroundings. The determined distances are signaled to a driver of the motor vehicle acoustically and with the aid of a display. Such a parking assistance system is known from DE 43 35 728 A1, for example.
A method for determining the distance uses a pulse echo method (propagation time measurements) of ultrasound pulses. The distance between the motor vehicle and an object is determined as the product of the speed of sound in air and a time period between transmission of an ultrasound pulse and receipt of the reflected portions of the ultrasound pulse. The time period is measured by a timer which is started by the transmission of the ultrasound pulse and is stopped at the point in time when a receiver receives an ultrasound signal which exceeds a predetermined signal level.